Linearization of a transmission chain in particular includes correcting deformation of the signal output from the chain.
Radiofrequency-domain power amplifiers normally generate distortions because they have non-linear responses. Applying compensation in power amplifiers introduces many constraints.
For example, the cost of technologies providing a satisfactory linearity, such as technologies produced on gallium arsenide (GaAs) substrates, is very high. Overspecing the amplifier, in a class-A design for example, increases the required supply current and means larger power modules are required, thermodynamic constraints appear, etc. However, this is incompatible with low consumption technologies, such as mobile telephones.
Furthermore, it is difficult to obtain a satisfactory linearity in high-modulation-order systems, in which the “peak to average” ratio of the emitted signals may be more than 10 dB, while maintaining a satisfactory efficiency in the transmission chain and the power amplifier. However, this is essential, in particular in terrestrial hertzian-wave emitter technologies.
Any nonlinearity limits the quality of service, for example in terms of data rate and bit error rate.
Furthermore, existing devices for predistorting the incident signal have limitations because of the need to associate a wide passband and a high incident-signal power.
FIG. 1 illustrates an example of a transmission chain DIS0 including a conventional predistortion device.
A digital block PR generates an incident baseband signal BBIN intended to be emitted over an antenna ANT at the end of the chain. The incident signal BBIN undergoes initial signal-shaping digital filtering in an input module TxDFE.
A digital predistortion module DPD generates and applies, to the incident signal BBIN, a predistortion signal depending on the incident signal BBIN and on the output signal RFOUT expected to be output from the chain.
The predistortion signal allows, by anticipation, deformation of the incident signal during its passage through the transmission chain to be compensated. Generally, the distortion in particular occurs in a power amplifier PA coupled to the antenna ANT.
The pre-distorted baseband digital signal is converted into an analog signal by a digital-to-analog converter CNA, then shifted by a radiofrequency modulator RFMOD, in particular by means of a mixer and a frequency shift signal LO generated by a local oscillator LOGEN.
The shifted signal is then amplified by the power amplifier PA and emitted by the antenna ANT.
A portion of the emitted signal is returned by a coupler CPL to a radiofrequency demodulator RFDEMOD using the same frequency shift signal LO, then converted into a digital signal by an analog-to-digital converter CAN, and transmitted to the digital predistortion block DPD.
There are various methods allowing the predistortion signal to be calculated, for example methods in which the “ideal” incident signal input into the chain and the distorted signal are compared by way of a look-up table.
Generally, these correction methods are limited to third-order modulation harmonics.
Fifth-order polynomial modelling methods also exist, in which an immediate error correction, obtained by comparing in real-time the emitted signal and the “ideal” signal, is applied.
However, the fifth order is not compatible with wideband channels, for example the channels of 56 MHz and 112 MHz widths of terrestrial hertzian-wave emitters.
Other linearization methods not necessarily requiring knowledge of the “ideal” signal input into the chain also exist. They require the stage upstream of the power amplifier PA, and in particular the modulator RFMOD, to have an excellent linearity.
In these types of correction methods and generally in conventional devices, the linearity correction is applied in the baseband domain and transmitted to the modulator via the digital-to-analog converter CNA.
Thus, the digital-to-analog converter CNA and the modulator RFMOD must at the same time meet the constraints of high bandwidth (to contain the spectrum of the predistortion signal) and of high power (the modulator RFMOD must be high-power to meet the needs of the transmission of the incident signal).
Therefore, the digital-to-analog converter CNA must have a high sampling frequency and a high input dynamic range. A high input dynamic range requires many unitary digital-to-analog conversion cells and therefore introduces substantial bulk.
Likewise, since the modulator RFMOD is driven by the pre-distorted signal, it operates at the power of the incident signal (i.e. several tens of decibels) and at the bandwidth of the predistortion signal (i.e. about three to five times the bandwidth of the incident signal).
However, it is problematic to achieve the bandwidth of the predistortion signal in the radiofrequency domain with large transistors.
Furthermore, it is desirable to produce transmission chain devices having a very good linearity while avoiding designs on gallium-arsenide substrates, in particular for reasons of production cost.